Would-be identity thieves no longer need bother rifling through dirty rubbish sacks. A major credit reference agency is linked to a promotion that is encouraging householders to leave personal data, including their dates of birth, in unsealed plastic bags on their front doorsteps.

Take one of these bags - brightly coloured, for extra visibility - and you could find out the names of the people living at the house, their mobile and home phone numbers, their monthly credit card balances, their occupations and salary levels, the problems they may have in paying back loans, and even the birthdays of their children. These, and many other equally intimate subjects, are included in the latest Household Insight Survey, which is being distributed across the country. The survey comes with a letter to householders that tells them to 'simply leave your completed survey in the bag provided on your doorstep'.

'Any criminal with a little bit of intelligence could walk down the street during darkness and gain an unhealthy amount of information about a lot of people,' says Michael Blakemore, whose own street in Durham was targeted for the promotion earlier this month. 'This way of collecting data is seriously risking identity theft.'

The Household Insight Survey, a questionnaire that encourages the public to voluntarily divulge personal information for marketing purposes, is operated by a subsidiary of TNT Post, the Dutch-based postal operator. TNT Post in turn supplies the data to a number of companies, including the market research and credit reference agency Experian. Ironically, Experian also offers a range of services designed to help individuals guard against identity theft.

Michael Blakemore, who, as a professor of geography at Durham University, has a professional interest in the use of marketing data for neighbourhood profiling, says he would have no problem with the survey if questionnaires were returned securely through the post. He says he is 'gobsmacked', however, at the lax collection methods that are being used.

'At a time when we are being exhorted to shred all personal information, this seems to drive a coach and horses through good practice,' he adds.

TNT Post remains unapologetic. Charles Neilson, the head of the company's Doordrop Media division, was unavailable for comment, but in a written statement TNT Post says its collection methods comply with data protection regulations. 'We believe this is a secure method of data collection,' the company asserts. TNT Post also defends itself by referring to the government agency that oversees data protection and privacy issues: 'TNT Post is properly and legally registered with the Information Commissioner's Office to perform these tasks,' it says.

In fact, TNT Post may find that its methods of data collection will shortly be coming under scrutiny by none other than the Information Commissioner's Office. Following an approach by The Observer, the ICO said that it would be taking up the issue with the company.

'It is clearly not best practice for organisations to ask people to leave forms containing personal information on the doorstep,' an ICO spokesperson says. 'We are living in an age when protecting your personal information has never been so important.'

The ICO has recently produced a 'toolkit' of advice for the public on ways to protect personal identity. It suggests storing documents carrying personal details in safe places and shredding or destroying any paperwork that carries your name and address. It also warns: 'Always think about who you are giving your information to.'

This advice is timely, because the latest figures from the anti-fraud trade association Cifas show that cases of successful identify theft increased by about 20 per cent last year, to around 68,000. Cases of attempted identity theft and impersonation also increased, to around 170,000.

According to TNT Post, it has carried out the doorstep collection of the Household Insight Survey for several years, and it has proved to be, in its words, a most effective method of data collection.

Experian, however, may be following the ICO in questioning this approach. 'As one of TNT's customers, we rely on the processes they use to collect their surveys. That said, if there are any concerns about security, rest assured we'll raise this with TNT and ask them to fully investigate the matter,' an Experian spokesperson said.

But there is one consolation for anyone who chooses to take the risk of putting valuable and sensitive personal information in a plastic bag on their doorstep. TNT Post offers in exchange ... a small bag of Thornton's chocolates.

How to beat ID theft

There are 14,000 victims of identity fraud each year. All that fraudsters need is your name and address to start forging your identity.

· Remember to shred (preferably with a cross-shredder) any documents that contain your name, address and any account details - this includes utility bills, bank statements or letters from your insurer.

· Check your bank and credit card statements regularly for alien transactions and try not to use the same passwords or Pins for all accounts. Once one code is cracked, all your accounts will be left open to fraud.

· When you move, redirect your mail to your new address by contacting the Royal Mail Redirection Service on 0845 7740 740 before you move - otherwise you may leave a paper trail of your personal details for thieves to follow.

· When shopping online, check the site is secure by making sure your browser displays 'https:' on the address line (or a padlock symbol somewhere on the window).

· If you bank online, always make sure you click the 'log out' button, rather than just closing the window. Never save passwords and make sure that your computer is protected with the relevant anti-virus software.